By-pass valve - CV vs Size

[sheiko]

Hello,

I am a junior process engineer working for an EPC company and we are in the detailed engineering phase. The basic studies have been performed by another company.

My problem is that i have the following list of CV and sizes for by-pass valves and i want to know wich manufacturer and which type of valve have been selected:
3/4" --> Cv=10
1/2" --> Cv=6.6
1" --> Cv=12
3/2" --> Cv=36

Furthemore i have heard of a scaling rule for geometrically similar valve that states: Cv=K*d^2 (K a constant and d the by-pass valve size). I have also heard that for gate: K=48, for globe: K=14.

Based on that and on the data provided during the basic studies, i have performed calculation that give me the following info:
3/4" --> K=18
1/2" --> K=26.4
1" --> K=12
3/2" --> K=16

My conclusion is that the 3/4", 1" and 3/2" by-pass valve give a K fairly close to K=14 and that they could be globe valve. Is it correct? Also what about the 1/2" by-pass vale?

Also could you please tell me if my approach is sensible?

P.S: sorry for my poor english as i am french

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[katmar]

It is customary to use a globe valve to give the regulating capability required for a bypass, and the Cv values you have given are typical for globe valves. A gate valve is more usually used as an isolation valve as it does not have a good characteristic for regulating. The 1/2" valve may have a slightly lower calculated Cv than would normally be expected for that size - but it just means the valve will not be used fully open.

Your scaling rule seems to come from the same basis as Equation 3-16 from Crane TP410, but the Crane formula is more complete. In Crane the "K" used is the same "K" as is used in flow resistance calculations for pipe fittings (we have had that discussion!!) and it is
Cv = ( 29.9*d² ) / ( K^0.5 )
where d is in inches

In your equation your "K" is equal to Crane's 29.9 / ( K^0.5 )

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[sheiko]

Thank you!

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."